Concrete Pour - Process?

[Crazy68Dart]

Hi all, I've read through some threads on garage floors. I am designing my garage, and have been talking with the GC. I've been asking some pretty specific questions, but am a little unclear on a "proper" prep/install for the concrete floor in the garage.

Size is looking roughly at 38x28. Pour will be 4" deep. GC's sub concrete guy uses a 4500 PSI mix, and wire mesh. I talked to the GC about insulation board and he said not problem, I could do that if I wanted. He also mentioned insulating the inside of the footer/block wall before backfilled to help insulate against the wall.

I don't want to be a PITA, but I am pretty picky and want it done right. I think the GC has a pretty good read on this based on the questions I have been asking.

Garage traditional stick frame, will be trenched/poured/rebar-reinforced footer then 5-6 course of blocks up to slight above grade. GC said I can add insulation against inside of blocks before backfilled. GC said blocks will be turned so that there is a ledge for the slab to rest on. How does this work if I want to add blue board under the whole thing? Does the insulation board sit on the block ledge?

What about vapor barrier? If insulation board is used, is this necessary?

I also might have two sets of 2x2' 8" footers poured for a future 2 post lift. How does the insulation board get setup in those areas? Or does it just get skipped in those small areas?

Sorry about all the questions... Thanks for the help.

 

[ConCretin]

I'm not sure what the climate is in NE Ohio but if you get any significant frost, I'd strongly recommend insulating the inside of the block wall before back filling - it will keep frost out of the soils under the slab.

To answer your specific questions;

Yes, the insulation should extend over the shelf and up the side of the slab. It can be beveled or thinner on the side if you don't want the top of the insulation to show as much.

Yes, you need a specific membrane type vapor barrier - 10 mil poly with taped seams and penetrations at a minimum. You could probably tape the insulation seams and get by but it wouldn't be a true vapor barrier.

Yes, you could fit insulation into the thickened areas for the lift but it won't make a noticeable difference in performance and could result in voids.

I'll also throw in my standard advice for slabs;

Make sure you have a stable, well compacted granular base that drains. The soils underneath the slab must support the loads placed on it.

Use properly supported steel reinforcing. Don't allow the finishing crew to pull it up as they go. Rebar is easier to support than mesh.

Use 4000 psi concrete and limit mix water with a mid range water reducer. The higher compressive strength isn't necessary but the higher water cement ratio will provide a more durable surface.

If a random crack, will ruin your day. Avoid re-entrant corners and use an early entry saw to cut control joints. Keep the panels as square as possible and cut 1/4 the depth of the slab.

Cure the slab by maintaining it in a continuously moist condition for at least seven days.

There is obviously more to it, but if you get these 5 points right, you're well on your way to a high performing garage slab.

Good luck with your project

 

[NZ0J]

I'm not sure what the climate is in NE Ohio but if you get any significant frost, I'd strongly recommend insulating the inside of the block wall before back filling - it will keep frost out of the soils under the slab.

To answer your specific questions;

Yes, the insulation should extend over the shelf and up the side of the slab. It can be beveled or thinner on the side if you don't want the top of the insulation to show as much.

Yes, you need a specific membrane type vapor barrier - 10 mil poly with taped seams and penetrations at a minimum. You could probably tape the insulation seams and get by but it wouldn't be a true vapor barrier.

Yes, you could fit insulation into the thickened areas for the lift but it won't make a noticeable difference in performance and could result in voids.

I'll also throw in my standard advice for slabs;

Make sure you have a stable, well compacted granular base that drains. The soils underneath the slab must support the loads placed on it.

Use properly supported steel reinforcing. Don't allow the finishing crew to pull it up as they go. Rebar is easier to support than mesh.

Use 4000 psi concrete and limit mix water with a mid range water reducer. The higher compressive strength isn't necessary but the higher water cement ratio will provide a more durable surface.

If a random crack, will ruin your day. Avoid re-entrant corners and use an early entry saw to cut control joints. Keep the panels as square as possible and cut 1/4 the depth of the slab.

Cure the slab by maintaining it in a continuously moist condition for at least seven days.

There is obviously more to it, but if you get these 5 points right, you're well on your way to a high performing garage slab.

Good luck with your project

This is all very good info.

I would like to add to the wire mesh info. If it were mine, I'd tell them to ditch the wire mesh and go with 3/8" rebar on chairs in your 4" slab. I can't tell you how many times I've seen concrete poured with wire mesh, and low and behold, the mesh gets left at the bottom of the concrete doing absolutely no good, and wasting money because it didn't get pulled up.

 

[Crazy68Dart]

Thank you, thank you, thank you. This is the recipe I have been looking for.

Which insulation board is everyone using? Does the vapor barrier go under the insulation board on top of the substrate?

I've seen a lot of slabs poured, non of which used a vapor barrier or insulation. Some wire mesh and some just fiber-mesh mix. In these cases, the slabs have been there for decades no cracks, etc. and have help up fine. Help me understand what is really important and what is being "****"?

Thanks again!

 

[hoho98925]

Most cracks are caused by improper soil prep. The sub-grade needs to be compacted well, then as others have said have the contractor soft-cut the slab- meaning cut it the same day it's poured no exceptions! Cracks often happen the night the concrete has been placed. As a general rule slabs should be cut in no larger then 400 sq ft grids. I cut mine in 10'x10 grids to control the cracks. Keep the slab wet or flooded especially in warmer weather, or you can use a curing compound (sealer) to keep in the moisture. Don't let the contractor take any shortcuts with your slab. As far as rebar and wire mesh neither on will do any good if not placed center of slab, it is ok to pull up wire mesh if the placing crew has one guy dedicated to just that. Good luck

 

[hoho98925]

I would also recommend pouring a 5" thick slab. For your size of shop that amounts to about an additional 3 yards of concrete about $300 depending on costs and it adds a substantial amount more strength.

 

[brownbagg]

you don't need a 4500 mix, thats a hot mix, not many residential finishers can handle a mix like that without problems. also u do need a vapor barrier

 

[Crazy68Dart]

This guys does a lot of concrete... his standard is 4500 PSI, it is not something I asked for specifically. I was surprised it was that high, myself.

Does the vapor barrier go under the insulation board, or on top of it?

Thanks.

 

[Diesel Dan]

If a random crack, will ruin your day. Avoid re-entrant corners and use an early entry saw to cut control joints. Keep the panels as square as possible and cut 1/4 the depth of the slab.

Why not use ZIP strip?

 

[ConCretin]

Why not use ZIP strip?

You could. It performs the same function as a saw cut. I find zip strip can be difficult to get in straight and can be a pain to deal with in the middle of a placement. It can be a good solution in radiant slabs if you're worried about cutting a tube.

 

[ConCretin]

Does the vapor barrier go under the insulation board, or on top of it?

It will perform it's function either way.

Some suggest that if you place the VB beneath the rigid, mix water will pool in the voids between the boards and recommend placing it on top. I'm not convinced this a significant issue but I suppose it's cheap insurance.

The advantage to placing the VB below the boards is that it is protected from damage. If you are stapling radiant tubing to the rigid, the VB has to go underneath or you'll poke it full of holes.

 

[mygarageone]

It will perform it's function either way.

Some suggest that if you place the VB beneath the rigid, mix water will pool in the voids between the boards and recommend placing it on top. I'm not convinced this a significant issue but I suppose it's cheap insurance.

The advantage to placing the VB below the boards is that it is protected from damage. If you are stapling radiant tubing to the rigid, the VB has to go underneath or you'll poke it full of holes.

Isn't foam board already a vapor Barrier ?

 

[Crazy68Dart]

Thanks again. It is obvious some of you have done this a few times.

How big of a deal is it if the insulation board is not tight to the walls? Are gaps okay, or do the board seams need taped? I know the vapor barrier needs to be taped. It seems to make more sense to me to put the barrier under the board, but who knows.

Does anyone have a insulation board recommendation? Install pics? (I am going to hunt around the gallery forum to see if I find anything)

Ignorance is bliss with some of this stuff.

 

[ConCretin]

Isn't foam board already a vapor Barrier ?

If you tape the seams, rigid insulation can act as a vapor barrier but if I recall correctly, it's permeance is greater than a membrane. The difference may not be enough to bother under most circumstances.

How big of a deal is it if the insulation board is not tight to the walls? Are gaps okay, or do the board seams need taped?

You want to keep the boards as tight as you can but gaps are inevitable. Taping isn't necessary unless you are relying on the insulation to be the VB.

 

[Crazy68Dart]

LLWillysFan, thanks for all the help and patience!

Same topic, different area... insulating against the inside of the block foundation before backfilled -- use the same insulation board that is used for under the slab? Is a vapor barrier used between the block and the board?

Would one expect to see the top of the insulation(i.e. thickness) around the perimeter of the slab? If it is running up the wall not sure how it would be avoided unless it stops flush with the board under the slab.

 

[cdestuck]

May I just add something in addition to your questions. Drains!! If your having drains put into the floor, spell out in the contract that water from all sides must drain into the drains or is coming out to be done correctly. I mess up on this when my floor was done and one area behind my wifes car runs back to the corner away from the drain. Never again will I over look this.

 

[ConCretin]

Same topic, different area... insulating against the inside of the block foundation before backfilled -- use the same insulation board that is used for under the slab? Is a vapor barrier used between the block and the board?

Would one expect to see the top of the insulation(i.e. thickness) around the perimeter of the slab? If it is running up the wall not sure how it would be avoided unless it stops flush with the board under the slab.

Yes, you can use the same insulation board. No vapor barrier is required below grade.

Vapor barriers have two primary uses. They are used in the walls of heated spaces to keep warm, moist air from reaching the point in the wall where the dew point results in condensation. Vapor barriers under slabs keep water vapor from migrating through the concrete. In addition to limiting unwanted moisture, it protects the adhesion of flooring products. Two different purposes, neither of which applies to below grade walls.

If you don't want the top of the insulation showing too much at the perimeter of the slab, you can use a thinner product or cut a bevel at the top of the board to minimize the thickness at the slab surface. You could use a closed cell foam expansion material that has a removable top strip that can be filled with caulk. Either way you'll achieve the desired thermal break.

 

[surfsup]

You want to keep the boards as tight as you can but gaps are inevitable. Taping isn't necessary unless you are relying on the insulation to be the VB.

Sorry for the response to an old post but did you insulate around your sump pump(s)? I plan to use some sort of foam stripping like a thick sill seal. Just curious what you did. Didin't see it listed anywhere...